A quantum version of Newton’s cradle was used to investigate how the chaotic motion of quantum particles eventually leads to thermal equilibrium. Using a laser-based cradle, researchers observed how, after inducing small amounts of chaotic motion, a quantum system reaches thermal equilibrium. Associate professor Benjamin Lev and his research team were inspired by the toy known as Newton's cradle in their investigation of quantum systems. Courtesy of L.A. Cicero/Stanford News Service. Researchers from Stanford University created a one-dimensional Bose gas with strong magnetic dipole-dipole interactions. A laser beam was shined through an airtight chamber to cool the gas to nearly absolute zero. The gas atoms were then loaded into an array of laser tubes that served as the structure for the Newton’s cradle. Each of the team’s 700 parallel cradles contained about 50 atoms. Researchers “kicked” the atoms with a laser beam to start the movement of the cradle. The Bose gas was strongly magnetic, which supported tunability. By tuning the interactions between the gas atoms, researchers were able to produce chaotic motion. Experimental results, confirmed by extensive computer simulation, showed that the cradles’ oscillation reached equilibrium in a two-step sequence: prethermalization followed by near-exponential thermalization. “It means we can have a very general, simple theory for how complicated quantum systems like this one thermalize,” professor Benjamin Lev said. “That’s beautiful because it allows you to translate that to other systems.” Researchers have several experiments planned for the magnetic quantum Newton’s cradle, and they anticipate additional opportunities for building upon this work as quantum engineering evolves. “Very sophisticated laser technologies can manipulate systems atom by atom,” researcher Yijun Tang said. “Maybe, at some point, we can turn these technologies into something more practical as well.” In experiments to come, the researchers may add disorder to the cradles’ tubes, in the form of speckled laser light, to see if they can create a sort of quantum glass that evades thermalization. The research was published in Physical Review X (doi:10.1103/PhysRevX.8.021030).